In the past several years, our work has concentrated in four distinct areas. Crystallographic studies of proteases Crystallographic studies of proteases have been an important area of research of this Section since its establishment. We have been particularly active in the investigation of structure-function relationship in aspartic proteases, including clinically important retroviral enzymes. Our studies of HIV protease, although no longer a major target of active research, are still ongoing and concentrate on the investigation of drug-resistant variants and their complexes with inhibitors. We have investigated retroviral proteases from several other sources such as FIV, RSV, and HTLV, having recently solved the structure of the latter enzyme. Cockroach allergen Bla g 2 was shown to be an inactive aspartic protease and we solved its structure in a complex with a specific antibody. We have established an extensive program of investigating serine-carboxyl peptidases (sedolisins), a family that was first characterized based on crystal structures solved in this laboratory and that is found in many different organisms. We are also investigating a bacterial ATP-dependent protease Lon, finding that is proteolytic domain has a unique fold and thus establishes a new family of proteases with a Ser-Lys catalytic dyad. We have solved the structures of the proteolytic domain of A and B type Lon proteases, encoded by E. coli and Archaeoglobus fulgidus, as well as the N-terminal and alpha domains of E. coli Lon. Lectins with antiviral activity We have been involved in studies of several lectins with antiviral activities, some of them currently being in pre-clinical trials as potential drugs preventing HIV infection. We have solved the structure of griffithsin, as free protein and complexed with a number of mono- and disaccharides, explaining the structural basis for its tight binding to branched mannose-rich carbohydrates. We have also solved atomic-resolution structure of another lectin, scytovirin. Proteins involved in ribosome biogenesis and tumor suppression Two related serine protein kinases, Rio1 and Rio2, are involved in processing 20S pre-RNA to 18S ribosomal RNA. Their crystal structures, solved by our Section, established that they belong to a novel family of kinases with a truncated substrate-binding region, although they are capable of both self- and trans-phosphorylation. We have established the sites of autophosphorylation for both of them. We are currently investigating their catalytic properties and a potential biological role. We have also solved the structure of the C-terminal MA3 domain of Pdcd4, explaining how that tumor suppressor factor inhibits translation initiation. Cytokines and cytokine receptors Our Section has been investigating the crystal structures of several cytokines and has made progress in preparing their receptor complexes. We have purified and crystallized complexes of IL-10 with its specific receptor and are studying complexes of several other cytokines related to IL-10, such as IL-19, IL-20, and IL-22. We are also investigating structural features of the interactions of IL-15 with the receptor molecules.